The invention relates to a system for noninvasive optical measurements of physiological properties in tissue. In particular, the invention relates to systems and methods comprising a light emitter emitting light of at least two different wavelengths, an optical detector, and a processor. The processor is capable of evaluating physiological properties from measurements of the optical detector.
Systems as mentioned above are widely used to evaluate and monitor physiological properties in tissue such as oxygenation of blood and heart rate (HR) in a subject and especially in a human body. In the context of this document, tissue designates a biological tissue. A biological tissue is a collection of interconnected cells that perform a similar function within a subject. Furthermore, the tissue can comprise at least a part of a vascular system where the vascular system comprises vessels (so called blood vessels). Vessels are for example arteries, capillaries or veins. When a tissue comprises blood, then the blood is always comprised in vessels of the tissue.
All known systems have the disadvantage that they are only capable of evaluating the physiological properties with a satisfactory quality in tissue of human bodies while the human body is kept at clearly defined conditions during the evaluation. These conditions or restraints require the tissue and/or the human body to be kept at rest or to be held still for most systems. These conditions or restraints may even apply to a time period before the evaluation, for example the need for resting before taking a blood pressure and/or an oximetry reading. Some other systems actively stimulate the tissue in a defined way (for example mechanically through vibration and/or application of pressure on the tissue) or require the tissue and/or body to perform clearly defined movements. During the stimulation or defined movements, the tissue and/or body have to fulfill respective conditions.
The reason for such requirements (i.e. such conditions or restraints) is that movement of the tissue or the body during the evaluation causes motion artifacts in the measurements. The same applies to the orientation of the tissue (i.e. whether the surface of the tissue is for example horizontally or vertically oriented). Changes in the orientation of the tissue lead to artifacts. These movement and/or orientation artifacts can for example be caused by a change in blood flow in the tissue or a shift in tissue layers and can be induced directly or indirectly to the tissue. Only clearly defined motions and/or orientations lead to measurements or variations of measurements which can be interpreted using the systems in the state of the art.
Systems with sensors which are attached to a fingertip and which measure light which is transmitted through the fingertip are known and widely used. These systems suffer from movement and/or orientation artifacts and other disadvantages as described above or they try to solve these issues by filtration of the measured signal after the measurement with complicated algorithms which produces a delay in the response.
It also has become known to use a mechanical sensor, such as an accelerometer, to determine whether the system is subject to motion by the user. The mechanical sensor is a sensor which measures mechanical forces, pressure and/or acceleration. Only data measured while the system is at rest are used.
A simple and reliable evaluation or monitoring of physiological properties of tissue by noninvasive optical methods is therefore not possible under real life conditions on moving subjects, especially when the subject movements occur constantly in irregular patterns. Throughout this text, subject stands for an organism and/or a body comprising the tissue which is to be measured. Especially, the subject can be a human body.